This application relates to an apparatus and method for transferring dry chemicals from a container, such as a railcar, and forming a solution or slurry of the chemical in a liquid carrier medium. The invention is particularly suitable for use with chemicals which form hydrates, in particular soda ash, and which therefore are difficult to handle using known transfer means.
Various means have been described for transferring dry materials out of containers. For example, U.S. Pat. No. 3,512,842 describes a method for unloading railcars in which a slurry is formed inside the railcar and then is pumped out. Such a process has numerous drawbacks, however, including the risk of overflowing or foaming within the car; the need for specialized railcars adapted for use in the slurrying process; the possibility of corrosive solutions being formed and damaging the railcar; problems with residual moisture causing caking in subsequent shipments; and the formation of hard, slowly dissolving lumps when liquid is added to a large quantity of solid. A similar approach is described in U.S. Pat. No. 4,189,262.
Eductors have been used and are still used to transfer dry chemicals as a slurry, solution or solid. For example, liquid driven eductors have been used to slurry dry polymers and activated carbon in the water treatment industry and to transfer fly ash in the electric power industry. Also air, steam, and liquid driven eductors have been used for transfer of solids. However, problems are known to exist with eductor-based handling systems.
For example, air driven eductors require a high power input and air flow per unit mass of solid conveyed resulting in high energy costs and higher capital cost for dust collection equipment. Steam driven eductors are used to create a vacuum for pneumatic conveying of dry solids to a solid-liquid mixing apparatus. The systems using steam driven eductors which are known to the inventors require a large amount of support equipment including a barometric leg for condensing the steam with modifications for solid-liquid mixing, and a large steam supply. Since the solid is conveyed by vacuum, the steam driven eductor system is limited by economics to installations where it can be located near, e.g. within a few hundred feet of, the container of dry chemical.
Liquid driven eductors do not require large volumes of air or steam and can be used to transfer dry chemicals from a container, such as a railcar, forming a solution or slurry of the chemical in the liquid carrier medium. Liquid driven eductors are known to be successfully used to prepare dilute solutions of polymer in water as well as to transfer insoluble materials, e.g. activated carbon, to storage as a slurry. However, the inventors are unaware of any liquid-driven eductor system used to transfer and dissolve or slurry dry hydratable solids when the motive liquid is a concentrated solution of the solid being transferred. In tests using concentrated solutions of a dry hydratable solid (i.e. soda ash) as the motive fluid to convey said solid, the throat of the eductor rapidly plugged with hydrates making frequent cleaning necessary. Also, tests using water as the motive fluid to convey a hydratable solid (e.g. soda ash) showed that plugging of the eductor occurred making cleaning necessary.
It is an object of the present invention to provide an apparatus and method for transferring solids, and particularly hydratable dry chemicals from a storage container, which avoids the plugging problems associated with known eductor systems. It is a further object of the invention to achieve this goal using a simple apparatus which is readily used in concert with conventional railcars.